Precision step voltage generator



Oct. 28, 1958 R. D. TOLLEFSON PRECISION STEP VOLTAGE GENERATOR :s Sheets-Sheet 2 Filed Sept. 25. 1956 Wk 4%; w

I INVENTOR. Roseer D. ToLLEAs'o/v BY v.

Mi /(A IQTTORNEYS Oct. 28, 1958 R. D. TOLLEFSON PRECISION STEP VOLTAGE GENERATOR 3 Sheets-Sheet 5 Filed Sept. 25, 1956 INVENTO ROBERT D. TbLLE/ BY 3 M 2 ,13 3.

[Q T TORNEYS' United States Patent Ofitice 2,858,434 Patented Oct. 28, 1958 PRECIION STEP VOLTAGE GENERATOR Robert D. Tollefson, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application September 25, 1956, Serial No. 611,977 7 Claims. (Cl. 250-27) This invention relates to means for obtaining substantial equality of steps in a stepped-voltage wave.

Prior stepped-voltage generating means do not generally maintain substantial equality among the incremental voltage amplitudes in their stepped-output. Often, their stepped increments decrease in value as the steps increase in absolute amplitude.

Therefore, the principal object of this invention is to teach how a precision stepped-voltage generator may be constructed.

It is another object of this invention to provide means for maintaining equality of amplitude among the incremental amplitudes of a stepped-voltage wave.

It'is still another object of this invention to provide a precise stepped-voltage generator that may be switched in a binary manner, wherein the number of output steps is large for a given number of components used in constructing the invention. I The invention includes floating-clamping means, comprising a pair of cathode followers, which maintain a given-reference potential between a pair of leads that float potential-wise with the stepped-voltage output. Current-controlling resistors are connected between the floating leads; and a common resistor is connected serially to one of the leads. A precisely stepped-output voltage may be taken from either lead.

The type of switching scheme used in a particular case of the invention determines the resistance values of the current-controlling resistors. Where a simple switching scheme is used which provides for each output step a respective switch and a respective current-controlling resistor, the resistors vary consecutively by inverse integer values and are switched in the same order. For example, the current-controlling resistors may 'be: 1 megohm, 500, 333, 250, etc. kilo-ohms.

However, the invention can be utilized more efliciently with a binary switching scheme. This arrangement permits a relatively small number of current-controlling resistors and switches, wherein N number of current-controlling resistors and switches can provide (2 1) number of output-voltage steps in each sequence of a repetitive wave. Furthermore, the binary switching scheme is easily provided electronically by a series of tandem-connected bistable circuits.

Further objects, features and advantages of the invention will be apparent to a person skilled in the art upon further study of the specification and drawings, in which:

Figure 1 illustrates aconventional stepped-voltage generating circuit utilizing a binary switching scheme;

Figure 2A, 2B, 2C and 2D illustrate a binary-switching scheme used with the respective switches in Figure 1;

, Figure 3 illustrates the lack of equality in the outputvoltage steps provided by conventional circuits;

Figure 4 is an illustrative form of the invention;

Figure 5 illustrates the output-voltage step sequence provided from the inventive circuit in Figure 4; and,

Figure 6 illustrates a form of the invention utilizing electronic switches.

In order to better understand the invention, a conventional circuit is illustrated in Figure 1, utilizing a binaryswitching scheme to provide a stepped-output voltage.- Illustrative values of components are given in order to assist the understanding. A pair of leads 10 and 11 are provided, with lead 10' being connected to a SO-volt lowimpedance direct-current power source having its other side (not shown) connected to ground. Four single-pole double-throw switches 8 through 8., have their poles respectively connected to lead 10. Four current-controlling resistors R R R and R are connected between the other lead 11 and a respective contact of each of the switches. They have respective values of one megohm, 500, 200, and 125 kilo-ohms. An output resistor R is connected between ground and lead 11 and has a value of 10 kilo-ohms in Figure 1. The stepped-output voltage is taken across resistor R Figure 2A through 2D illustrate the time-repetitive switching sequence for the four switches in Figure 1. Hence, switch S opens and closes according to Figure 2A, switch S according to Figure 2B, switch S according to Figure 2C, and switch 5., according to Figure 2D.

The four switches in Figure 1 permit a maximum of fifteen voltage steps in each stepping sequence. The stepping sequence is, of course, repetitious as long as the switches are operated continuously according to Figures 2A through 2D.

As a result of this sequence of opening and'closing the switches, the stepped-output voltage in Figure 1 varies approximately as illustrated in Figure 3, where it can be seen that the voltage steps vary in a non-linear manner to follow approximately a non-linear defining curve 12.

It is therefore noted in Figure 3 (which only shows eight of the fifteen step sequence) that much incremental variation is obtained among the voltage steps provided by the circuit of Figure 1. For example, the first step is 0.499 volt, while the eighth step is 0.31 volt.

The invention teaches how such voltage steps may be made equal, and Figure 4 illustratively applies the invention to the circuit of Figure 1 to provide substantially equal output voltage steps.

In Figure 4 a first plurality of diodes 21 through 24 are respectively connected in series with resistors R through R and have their anodes respectively connected to the poles of'switches S through S Another plurality of resistors R through R are each illustrated as kilo-ohm resistors in Figure 4. Theyare respectively connected between the poles of switches S through 8., and a 100-volt direct-current power source, which has its other side (not shown) connected to ground. I A second plurality of diodes 31 through 34 also have their anodes respectively connected to the poles of switches S through 8., but have their cathodes connected to lead 10.

Switches S through 8., act as shunting devices in Figure 4 rather than as the series devices of Figure 1. Thus, in Figure 4 each switch has one of its contacts connected to ground and its other contact unconnected. The ground connection of the respective switches S through 8., may instead be connected to a negative-potential source which can in some cases improve the cutoff bias on the diodes.

The illustrated positions of the switches in Figure 4 connect the anodes of all of the diodes to ground to maintain them in a non-conducting state. Moving the pole of any switch to its other contact will bias its respective diodes into a conducting condition, because their anodes are then raised to a positive value by a voltage divider D provided between the 100-volt power source and ground by the respective series resistors R R and R A pair of electron-control means V and V are connected as cathode followers, and are illustrated as triodes. Triodes V and V have their plates directly connected to the positive direct-current power source which is +180 volts in Figure 4. Both cathode followers operate in their substantially linear region between cutoff and grid-current limiting. Thus, a cathode resistor 41 is connected between the cathode of tube V and a negative directcurrent power source illustrated to provide -75 volts. The negative cathode supply is required for cathodefollower V to maintain it in its region between cutoff and grid-current limiting. The grid of tube V, is connected to lead 11.

Cathode follower V is operated at a higher grid-voltage level than follower V which permits follower V to have its cathode resistor 4-52 connected between the groundpotential level and the cathode of tube V Lead 10 connects to the cathode of follower V to receive its output.

A fixed-reference voltage E is provided between the grid of follower V and the cathode of follower V A voltage-regulator tube 43 maintains voltage E at a fixed value. A pair of resistors 46 and 47, comprising a voltage divider, are connected across regulator tube 43, and a resistor 4-8 is connected between the anode of tube 43 and the +180 volt supply. Resistor 46 has one end connected to the grid of tube V and its other end connected to the cathode of tube V This example assumes that reference voltage E is +50 volts.

The electron-control means V and V are chosen to have as high an amplification factor as possible, in order to provide the cathode followers with substantially unity voltage gain. Thus, the voltages on the cathode and grid of tube V will be substantially equal; and similarly, the voltages on the cathode and grid of tube V will be substantially equal. A high amplification factor also provides a low output impedance for cathode follower V that for example may be of the order of two-hundred ohms.

The switching scheme for switches S through 8,, in Figure 4 is the same as described for Figure 1, and is illustrated similarly by Figures 2A through 2D. Consequently, the binary switching scheme provides up to a maximum of fifteen output steps since N is four in the limiting formula (2 1).

The output voltage E in Figure 4 across resistor R varies with substantially equal steps as illustrated in Figure 5, wherein each step is substantially one-half volt. Thus, a linear defining line Sll may be drawn in Figure 5.

In operation, the invention maintains a constant voltage, equal to the reference voltageE between leads 10 and 11 throughout the stepping sequence. Thus, in Figure 4,

lead 10 is always maintainedat +50 volts above lead 11, and provides the same stepping sequence as lead 11, but at a 50-volt higher level.

The voltages on leads l0 and 11 float with the varying output voltage E due to the following operation of cathode followers V and V Output voltage E is applied directly to the grid of tube V because it is connected to lead 11. Hence, the Voltage on the cathode of tube V will be very nearly equal to output voltage E because of the unity voltage gain of cathode follower V Accordingly, the instantaneous voltage on the grid of tube V is (E +E because reference voltage E is there added to output voltage E of tube V by the connection of the reference-voltage source.

Since the voltage on the cathode of tube V is substantially equal to the voltage on its grid because of the unity voltage gain, its output voltage E is defined by the following formula:

Therefore, the instantaneous voltage on lead 10 is (E +E while the instantaneous voltage on lead 11 is E0.

While switches S through 8, are in the grounded positions shown in Figure 4, output voltage E is zero, because both ends of resistor R are connected either directly or indirectly to the ground potential level. Thus, there will be no current output provided from cathode follower V because diodes 31 through 34 are biased below cutoff by having their anodes grounded while their cathodes are at the positive potential of the cathode of tube V The respective voltage dividers B, through D, are provided by ungrounding the respective switches. Whenever any switch is moved to its ungrounded position, its diode of 31 through 34 is thus biased to a conducting state by its respective voltage divider D, since the positive voltage provided at its switch pole is at all times more positive than the output voltage of cathode follower V Furthermore, it is more positive than voltage E provided on the other side of its diode of 21 through 24 to maintain it conducting. Therefore, the voltage-divider values of resistors R and R are chosen so that when all switches are ungrounded, the positive potential provided by the respective voltage dividers to their diodes always exceeds voltage E from tube V This insures that the diodes are always in a conducting state whenever their respective switches are ungrounded.

In the simplest of the binary switching situations, only one of the switches 8; through 5., is ungrounded. This provides a circuit between ground and the l00-volt source through the respective voltage divider and connects cathode-follower voltage E through the low forward impedances of the respective diodes to its resistor R which is in series with output resistor R A current I flows through the series-connected resistors R and R and is caused by the cathode-follower output voltage E being maintained across them. Current I may therefore be defined by the expression:

RN+R0 Also, in this case, output voltage E may be defined by the expression:

In Expression 4, both R and E are constants. As a result, the system of Figure 4 allows the output-voltage E provided across resistor R to be controlled by a single set of parameters, which are the values of resistors R In order to theoretically obtain equal-voltage steps across a common resistor that has currents switched through it in a binary fashion, it is necessary for the respective currents to have amplitudes that are binary related, wherein the adjacent amplitudes double in value as the adjacent switching rates decrease by one-half. Thus, to provide the proper current relationships through resistor R for the binary switching scheme, there must be a binary selection of resistors R wherein any two resistors in the sequence have values with 2-to-1 ratio.

The general case for a stepped-output voltage using a switching scheme, such as the binary one, where simultaneous conduction of plural resistors R is required during parts of the sequence, is given by the expression:

decade number of steps can be obtained with four resistors R by causing the switching scheme to reset to zero after the ninth step.

Other switching sequences than binary may be used in the invention, and the above Expressions 1 through apply. For example, the simplest sequence provides one output-voltage step for each resistor R This straightforward switching scheme is, however, not usually the most efiicient. It has its output defined completely by Expression 4. In this case, the values in a sequence of resistors R vary consecutively in inverted-integer fractions of the first resistor, in the order 1,' V2, /3, A, etc.

A slight source of error can exist in the invention due to the output impedance of the cathode follower. However, this error can be made a small fraction of one percent by having the cathode-follower output impedance very small compared to resistors R and having resistors R approximately the same order as resistors R The binary switching scheme explained above is also particularly useful because it can conveniently utilize electronically-controlled bistable circuits, such as multivibrators, in the well-known fashion of binary counters.

Figure 6 .shows such a switching system and comprises a plurality of conventional bistable multivibrators 61 through 64 which are driven by a pulse generator 60 connected to the input of first multivibrator 61. The multivibrators are conventionally coupled by capacitors 66.

Each multivibrator is connected between a positive direct-voltage supply B+ and a negative direct-voltage supply 0- to provide an output voltage that switches between a low level that is the ground-potential level (or slightly below) and a high level that might for xeample be 100 volts. With multivibrators, it is preferable that the potential level be slightly below the ground potential level. If the multivibrators also provide an output impedance of 100 kilo-ohms, they switch analogously to switches S through 8.; with their respective resistors R in Figure 4.

Although the bistable multivibrators may each have two opposite outputs, the output is selected which correlates with the switching sequence described in Figure 2; wherein at the beginning of a sequence of steps, all of the multivibrator outputs simultaneously begin a period at their lower-voltage level.

Except for the replacement of switches S through 8,, and their resistors R through R by respective bistable and their resistors R through R;;, by respective bistable multivibrators 61 through 64, Figure 6 is otherwise identical in construction to Figure 4, and its operation is explainable in the same manner.

Although this invention has been described with respect to particular embodiments thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims.

I claim:

1. Means for generating a sequence of incrementallyvarying voltages comprising a plurality of switching means, ground-potential means, each of said switching means providing a pair of voltage states, with one voltage state being at or below ground potential, and the other voltage state being at positive potential, a positive source of direct voltage, output-resistance means having one end connected to said ground-potential means, a plurality of current-controlling resistance means, a first plurality of asymmetric conductors, each being connected in series with one of said current-controlling resistance means between the other end of said output-resistance means and one of said switching means respectively, a pair of cathode followers, each operating linearly over the range of its input voltages, the input of the first cathode follower being connected to said other end of said output-resistance means, a reference-voltage source being connected between the input of the second cathode follower and the cathode of the first cathode follower, and a second plurality of asymmetric conductors connected between the cathode of said second cathode follower and said switches respectively, with the asymmetric conductors being biased above cutoff only by the positive-potential state of its respective switching means.

2. Means for generating a sequence of incrementallyvarying voltages comprising a plurality of switching means, each of said switching means alternately providing a low potential output and a high potential output, a common lead, an output resistor connected between ground and said common lead, a plurality of current-controlling resistance means having resistances related by a geometric progression, said resistance means being individually connected in series between said common lead and the respective individual switching means, a first plurality of diodes connected between said common lead and said switching means respectively, first and second cathode followers operating in their linear regions, a negativevoltage source providing the cathode current return of said first cathode follower, and said second cathode follower having its cathode-current returned to ground potential, the control grid of said first cathode follower being connected to said common lead, a second plurality of diodes connected between the cathode of said second cathode follower and said switching means respectively, a fixed-potential source connected between the control grid of said second cathode follower and the cathode of said first cathode follower, said fixed source being independent of ground potential, the diodes respectively connected to each of said switching means being alternately maintained in conducting and non-conducting conditions by its opposite switching states, and said current-controlling resistors having their values selected according to the switching of said switching'means.

3. A circuit as defined in claim 2 in which a voltageregulating tube comprises said fixed-potential source.

4. A circuit as defined in claim 3 in which a binary switching scheme is utilized, wherein said switching means are switched on and off with binary time-division relationships, and said current-controlling resistors having binary values, wherein each is related to its adjacent resistor by a two-to-one resistance ratio.

5. A precision step-voltage generator comprising a plurality ofbistable circuits connected in tandem, with each bistable circuit providing an output having alternately a high potential state and a low potential state, first and second leads, a first plurality of diodes connected between said first lead and the respective outputs of said bistable circuits; a plurality of series circuits, each comprising a diode and a current-controlling resistor, said series circuits connected between said second lead and the respective outputs of said bistable circuits, with the resistors in said series circuits having their resistances proportioned in a binary decreasing fashion, output resistance means connected between said second lead and ground, first and second cathode followers operating linearly, the cathode of said second cathode follower being connected to said first lead, the grid of said first cathode follower being connected to said second lead, and a constant voltage source being connected between the grid of said second cathode follower and the cathode of said first cathode follower.

6. Precision step-voltage generating means comprising a pulse generator, a plurality of multivibrators connected in tandem with said pulse generator, with the outputs of said multivibrators being switched in binary-division relationship; a C-minus source, and a B-plus source being connected to each multivibrator to establish its opposite direct-current levels; each multivibrator having an output that switches between a high potential level approximately equal to the B-plus source and a low potential level equal or below the ground-potential level of the system, a first cathode follower having its plate connected to said B-plus supply and its cathode current returned to said C-minus supply, an output resistor connected between the grid of said first cathode follower and ground, a second cathode follower having its plate connected to said B-plus supply and having its cathode current returned to the groundpotential level, a first plurality of diodes, a plurality of current-controlling resistors, each connected in series with said output resistor and one of said first plurality of diodes and the output of one of said plurality of multivibrators, a second plurality of diodes respectively connected between the outputs of said multivibrators and the cathode of said second cathode follower, a constant-voltage source connected between the grid of said second cathode follower and the cathode of said first cathode follower, and the diodes connected to any of said multivibrator outputs being biased to a conducting state by its high-potential output and being biased to substantial nonconduction by its low-potential output state.

7. Means for stabilizing step voltages comprising a plurality of bistable circuits connected in tandem, each of said bistable circuits having an output alternately being at a high-voltage state and a low-voltage state, said low-voltage state being at or below ground potential, first and second common leads, output resistance means having one end connected to ground and its other end connected to said first common lead, a plurality of current-controlling resistors, a first plurality of diodes connected in series with said output resistor and said current-controlling resistors respectively and the outputs of said bistable circuits respectively, a second plurality of diodes being connected between said second common lead and the outputs of said bistable circuits respectively, with any of said second plurality of diodes being biased to a non-conducting state by receiving said low-voltage output and being biased to a conducting state by receiving said high-voltage output, a first cathode follower having its grid connected to said first common lead, a negative-potential source providing the cathode current return for said first cathode follower to permit linear operation, a second cathode follower, a B-plus source connected to the plates of said first and second cathode followers, said second cathode follower having its cathode current returned to ground potential, with its cathode being connected to said second common lead, and a constant-voltage source connected between the grid of said second cathode follower and the cathode of said first cathode follower.

References Cited inthe file of this patent UNITED STATES PATENTS 

